Satellite communication systems have several applications. For example they are used in the field of television, mobile telephony, and terrestrial and sea navigation. In such systems, the exchange of information is based on the propagation of a signal via satellite connections, which cover long distances without the use of particular infrastructures along the covered distances.
For example, satellite television is a system consisting of an artificial satellite able to receive a television signal from one or more transmitter stations and directly retransmit it to different users. Each user is provided with one or more receiving antennas (such as parabolic antennas) which are directed towards one or more satellites, which antennas receive the television signal, perform a first conversation/selection of the high-frequency satellite signal and send it to a decoder (Set-Top Box) to which they are connected by a coaxial cable. The decoder is able to process the received television signal so that it can be exploited by the user through normal television apparatuses. The decoder forms what is referred to as the Internal Decoding Unit (“IDU”), whereas the receiving antennas form the Out Decoding Unit (“ODU”).
In digital satellite television, the television signal processed by the decoder is in particular a digital signal.
In some application, the decoder interacts with each receiving antenna using a communication protocol which requires the superimposing of a low frequency signal (for example, at 22 KHz) with a pulse width keying on a DC signal forming the supply voltage, which is provided to each receiving antenna by the decoder itself.
In order to guarantee the correct exchange of information between each receiving unit and the decoder, it is necessary that the low frequency signal complies with particular requirements. For example, in the case in which the low frequency signal is a square wave, it is required that the amplitude thereof, the rising and falling time, the duty-cycle thereof (defined as the ratio between the time in which the signal is at the high voltage level and the period of the signal) take predetermined values.
For this purpose, the decoders have regulation and shaping circuits adapted to provide the low frequency signal having the desired features.
Examples of regulation and shaping circuits of the low frequency signal are known in the art.
In FIG. 1 an example of a conventional regulation and shaping circuit 100 of low frequency signal of the above-mentioned type is shown.
The shaping and regulation circuit 100 receives in input an input voltage signal Vin and provides in output an output voltage signal Vout adapted for being used as a low frequency signal.
For this purpose, the shaping and regulation circuit 100 comprises an input terminal 105 which receives the input voltage signal Vin, and an output terminal 110 which sends to a coaxial cable 115 to which it is connected the output voltage signal Vout. In the example at issue, the input voltage signal Vin consists of a square wave oscillating between a first predetermined voltage Vin′* and a second predetermined voltage Vin″*, with a frequency f1 (for example, 22 kHz). Typically, the first predetermined voltage Vin′* and the second predetermined voltage Vin″* take positive values so that also the output voltage signal takes positive values (indeed, the communication protocol between the decoder and each receiving antenna provides for the use of voltage signals which take values higher than zero).
In detail, the regulation and shaping circuit 100 includes a differential amplifier 120 having a relatively high gain (for example, ranging between 60 dB and 80 dB). The differential amplifier 120 has an inverting input terminal (denoted in FIG. 1 with the symbol “−”) and a non-inverting input terminal (denoted in FIG. 1 with the symbol “+”). The differential amplifier 120 receives as power supply a reference voltage, for example ground, and a power supply voltage Vdd (for example, 20V). Typically, the supply voltage Vdd is higher than the maximum voltage value which can be reached by the output voltage signal Vout. The non-inverting terminal of the differential amplifier 120 is connected to the input terminal 105, whereas the inverting terminal is connected to a first terminal of a resistor R1, which has a second terminal kept to ground. The resistor R1 has the first terminal connected to a first terminal of a resistor R2, which has a second terminal connected to the output terminal 110 of the regulation and shaping circuit 100. An npn bipolar transistor T1 is connected between an output terminal 125 of the differential amplifier 120 and the output terminal 110 of the regulation and shaping circuit 100. In particular, the transistor T1 has an emitter terminal connected to the output terminal 110, a base terminal connected to the output terminal 125 of the differential amplifier 120 and a collector terminal, which receives the supply voltage Vdd.
The regulation and shaping circuit 100 provides for the use of a negative feedback control loop (comprising the differential amplifier 120, the transistor T1, the resistor R1 and the resistor R2) for controlling and setting the output voltage signal Vout. In particular, the differential amplifier 120 is connected to the network formed by the resistors R1 and R2 and by the transistor T1 according to a conventional non-inverting configuration.
In response to the input voltage signal Vin (applied to the input terminal 105) the regulation and shaping circuit 100 provides at the output terminal 110 the output voltage signal Vout having a square shape wave, whose amplitude is a function of the resistance of the resistors R1 and R2. In particular, the output voltage signal Vout has an average value substantially equal to the supply voltage to be provided by the decoder to each receiving antenna and it is used as a low frequency signal (with a dynamic range of the order of some thousands of mV) during the exchange of information between the decoder and each receiving antenna.
A drawback of such solution is that for the output voltage signal Vout not to be distorted (thereby keeping the amplitude needed for the correct transmission of the signal), it may be necessary to provide an output circuit (for example, of push-pull type) connected to the output terminal 110. This causes an increase of the area occupied by the regulation and shaping circuit 100 in a semiconductor chip in which it is integrated. On the other hand, to guarantee the integrity of the wave shape of the output voltage signal Vout it may be needed to increase the voltage drop at the transistor T1, thus increasing the circuit consumption.
Moreover, the frequency of the output voltage signal Vout depends on the pass band of the regulation and shaping circuit (with negative feedback) 100. In particular, the maximum obtainable frequency of the output voltage signal Vout is limited by the pass band of the regulation and shaping circuit 100. This can cause an undesired shift of the frequency of the output voltage signal Vout from the provided one.
FIG. 2 shows another conventional regulation and shaping circuit 200. The regulation and shaping circuit 200 receives in input a first reference voltage Vref and an input voltage signal Vin1, and provides in output an output voltage signal Vout1, consisting of a low frequency signal, superimposed on a DC voltage.
For this purpose, the regulation and shaping circuit 200 includes a first input terminal 205, through which it receives the first input reference voltage Vref, a second input terminal 210 through which it receives the input voltage signal Vin1, and an output terminal 215. In the example at issue, the first reference voltage Vref takes a constant value for example, 1.25V; the input voltage signal Vin1 consists of a square wave ranging from a first voltage value V1 (for example, the ground) to a second voltage value V2 (for example, 1V) with the frequency f1 (for example, 22 kHz).
The regulation and shaping circuit 200 includes a regulation circuital structure 100′ similar to that of the regulation and shaping circuit 100 (for this reason similar elements are denoted with the same reference numerals with the addition of an apex), which is connected to the input terminal 205. In this case, the emitter terminal of the transistor T1′ is coupled to the output terminal 215 by means of an impedance Z1. In particular, the impedance Z1 has a first terminal connected to the emitter terminal of the transistor T1′ and a second terminal coupled to the coaxial cable 115; moreover, the impedance Z1 consists of an inductor L1 which is connected in parallel to a resistor R3.
An n-channel MOSFET T2 has a drain terminal, which is connected to the output terminal 215, a control terminal, which is connected to the second input terminal 210, and a source terminal, which is connected to a first terminal of a resistor R4. The resistor R4 has a second terminal kept to ground.
During the operation of the regulation and shaping circuit 200, the output voltage signal Vout1 is obtained by superimposing on a DC output voltage signal due uniquely to the first reference voltage Vref (when the second input terminal 210 is kept to ground), a low frequency voltage signal due uniquely to the input voltage signal Vin1 (that is the signal which it should be observed at the output terminal 215 when the first input terminal 205 is at ground).
When the regulation and shaping circuit 200 receives only the first reference voltage Vref and the input voltage signal Vin1 is kept to ground, the transistor T2 is turned off so that no current flows in the circuital branch formed by the transistor T2 and by the resistor R4. The output terminal 215 reaches a constant value which, similarly to the case of FIG. 1, depends on the value of the first reference voltage Vref and on the ratio between the resistances of the two resistors R1′ and R2′ (for very low frequencies, the inductor L1 has a very low resistance, ideally it is a short-circuit, so that the voltage reached by the emitter terminal of the transistor T1′ is directly transmitted to the output terminal 215).
When the regulation and shaping circuit 200 receives only the input voltage signal Vin1 and the first reference voltage Vref is kept to ground, the regulation circuit 100′ is inhibited, whereas the circuital branch formed by the transistor T2 and by the resistor R4 is conductive. In particular, in response to the input voltage signal Vin1, the output terminal 215 provides an output voltage signal having a square wave shape with a frequency equal to the frequency of the input voltage signal Vin1.
In particular, the input voltage signal Vin1 generates in output a signal having a small amplitude with respect to the DC value. Therefore, the output voltage signal Vout1 is formed by a DC voltage value—for example, ranging from 13V to 18V—effect of the first reference voltage Vref− to which a small signal of 22 KHz is superimposed having a peak-to-peak amplitude significantly lower than the DC voltage value (for example, approximately 700 mV).
In other words, the output voltage signal Vout1 obtained by the superposition of the effects of the first reference voltage Vref and the input voltage signal Vin1, consists of a square wave having a frequency depending on the frequency of the input voltage signal Vin1 and average value function of the first reference voltage Vref. In particular, such average value corresponds to the DC supply voltage to be provided to each receiving antenna, whereas the output voltage signal due to the input voltage signal Vin1 is adapted to be used as a low frequency signal for communicating with the receiving antennas.
It should be noted that the use of the transistor T2 implies a lower control of the wave shape of the output voltage signal Vout1. Indeed, the lack of a negative feedback may prevent stabilizing the output voltage signal Vout1 and thus the low frequency signal.
Moreover, the amplitude of the output voltage signal Vout1 depends not only on the values of the resistance of the resistors R1′ and R2′, but also on the impedance Z1 and on the impedance of the coaxial cable 115′, so that it may not be possible to obtain an output voltage signal Vout1 having an amplitude equal to the desired one.